The diagnosis of mast cell lesions of the skin can occasionally be challenging. Calretinin, a 29 kD neuron-specific calcium-binding protein found mostly in the CNS and retina, has been shown to be a positive marker for mesotheliomas, and is also expressed in mast cells. We studied the diagnostic value of calretinin and compared our results to other established ancillary studies used to identify mast cells, such as Toluidine blue and the Leder stain. Sixty-three cases were studied, including 45 mast cell lesions (22 urticaria pigmentosum, 17 mastocytomas, and six telangiectasia macularis eruptiva perstans [TMEP]), seven nevi, three melanomas, four granular cell minors of the skin, three cutaneous lymphomas, and one granulocytic sarcoma. Patients ranged in age from less than 1 to 85 years with a median age of 29 years. The group consisted of 36 females and 27 males. Calretinin was expressed in all 45 mast cell lesions. Negative staining for calretinin was seen in all skin lesions that potentially could be considered in the differential diagnosis of mast cell lesions such as nevi, melanomas, lymphomas, and the granulocytic sarcoma. However, calretinin expression was noted in four/four granular cell tumors. Leder and Toluidine blue stains were positive in all 45 mast cell lesions, and all nonmast cell lesions were negative with these stains. In conclusion, our study demonstrated that calretinin is a sensitive and specific marker of mast cells and can be an aid in distinguishing mast cell lesions from other skin lesions considered in the differential diagnosis. Calretinin may be more sensitive than the currently used special stains utilized to diagnose mast cell lesions having few diagnostic mast cells such as TMEP. However, this immunoperoxidase stain does not add significant diagnostic information in most cases, when compared with the currently used less expensive special stains and, therefore, is not cost-effective. Int J Surg Pathol 8(2):119-122, 2000